Hippocampal (HC) neurons exist in two populations based on initial intracellular pH (pHi); one consisting of neurons with low initial pHi values and the other with high initial pHi values. High-pHi neurons recover faster from acid loads than do low-pHi neurons. There are two specific aims: (1) to explore the molecular basis of low- vs. high-pHi neurons and determine whether the difference relates to neurotransmitter content and/or properties of specific HCO3 transporters expressed by a neuron. The approach will be to monitor pHi in HC neurons grown on gridded coverslips and assess activities of HCO3 transporters in identified cells. Immunocytochemistry and single-cell PCR will be used to assess molecular phenotype in the same cells. A knockdown technique will show how the expression of specific HCO3 transporters affects the low/high pHi state. (2) To use chronic hypoxia, and the return to normoxia, as tools to shift the low- vs. high-pHi status of HC neurons. The approach will be to monitor individual neurons. The results from these studies are significant because the pHi differences between low- and high-pHi neurons are large enough to have a major impact on neuronal activity. In other words, pHi shifts could be a form of neuromodulation.